From x360achievements:
This is done on the 3rd mission "Running with the Wolves...". Now you will make a total of 3 stops (including where you have to 'Eliminate enemies with M82") before you come to the spot to get this achievement. When you get on the ATV the fourth time, start driving for a while. You will eventually have to drive up a big hill, and then down the big hill when you reach the top. Once you reach the bottom you will have to take a left and then there will be 2 big drops in a row (2nd drop is seconds after the 1st). This is the area you will get the achievement. You want to make sure there is a decent gap between you and "Dusty" so that you can go full speed without bumping into him. As long as you drive fast and hit the drop/jump head on, you will unlock this achievment. If you go off both drops but don't get the achievement after a few seconds, restart checkpoint and try again. The spot is right before you rally up with your team.